In such blades, it is known that the bundle of continuous filaments may advantageously be constructed in the form of a plurality of bands attached and agglomerated to one another, of unidirectional and longitudinal rovings (i.e., which extend along the longitudinal axis or the span of the blade), which are agglomerated by a thermosetting and polymerized impregnation synthetic resin, and which are wound around the sleeves in such a manner as to constitute a fastening loop.
The fastening device including sleeves according to the invention can be used in a general way in the field of construction, and especially of aeronautical construction, in order to ensure the connection, to a support, of a composite filamentary component, subjected to very large traction loads and in which the sleeves are integrated, by means of an axle or of an arbor traversing the sleeves. In other words, to construct the inner ring system for absorbing forces of a bore of a composite filamentary element which is to be connected to a support by a mechanical connecting axle traversing the ring system and subjected to large radial forces, the fastening device including metallic sleeves according to the invention will be described exclusively in its application to the connection of the rotor blades of rotor aircraft to the hub or to a component for connection to the corresponding hub, when the blades are constructed of composite material and when their roots have at least one loop formed by a winding of rovings. In this field of application the fastening device according to the invention appears to provide the greatest benefit, by reason of the magnitude of the traction centrifugal force which urges the fastening of each blade in rotation onto the hub and which ranges from 10,000 to 40,000 daN for current helicopters, depending upon their size.
In order to connect the root of such a helicopter rotor blade to its hub, by attaching it, for example, to a rigid support strap exhibited either directly by the hub or by an armature shaped in the form of a double strap or ferrule for connection to the hub, use has already been made of fastening sleeves of biconical external shape, and each traversed axially by a cylindrical bore for accommodating the retaining arbor or axle. The advantages of these biconical sleeves were, on the one hand, that of facilitating the closure on one another of the upper surface and lower surface half-molds cooperating and assembled to constitute a mold for polymerization of the blade constructed of composite material in the course of its production and, on the other hand, that of permitting a definitive capture of each biconical sleeve in the corresponding blade root to be obtained. As long as the rovings forming the loop of the blade root were placed in position individually and successively in the mold, it was possible to smooth them by hand one by one, in order to compensate, little by little, for the differences in length over the rovings joined at the biconical outer surface of the sleeves. However, following the appearance and the development of the techniques of utilizing rovings in a hank and of mechanical windings of rovings in the form of bands, capable of forming the hanks, it was no longer possible to maintain on the sleeves winding surfaces of conical form; the led to having rovings tight on an edge of a band (on the side of the large base of a conic section) and corrugated on the other edge of the band (on the side of the small base of the same conic portion). Moreover, the bands of rovings naturally had a tendency to escape from the periphery of the conic portions, progressively as the thickness of the stack of bands increased. These major disadvantages made it necessary to abandon the biconical sleeves, and led to the use of cylindrical sleeves.
Numerous examples of connection of helicopter blades to the corresponding rotor hubs by means of metallic and cylindrical sleeves for absorbing forces have already been proposed.
In French Pat. No. 1,544,344, which relates to a device for anchoring an element constructed of synthetic material reinforced with glass fibers and terminated by a loop, such as a rotor blade, the root of which is formed by the loop, in a metallic armature formed of two half-shells which entirely captively retain the loop, the latter is formed by means of hanks of rovings extending as far as the spar or the end of the blade, and surrounds a cylindrical sleeve which is embedded in the armature, and which is traversed by an axle for fixing to the hub of the rotor.
A contact layer, for example of fluorocarbon resin, exhibiting a high degree of resistance to shocks and to compression, a low coefficient of friction, and therefore good anti-adhesive properties and a good resistance to wear is spread, in order to reduce friction effects, between, on the one hand, the composite loop and, on the other hand, the metallic armature and the sleeve, in order to avoid the development of corrosion by friction and damage to the blade root by permitting free deformations of the blade root and of the armature independently of one another, depending upon their own elasticity modulus. In this construction, affixing is therefore avoided between the loop and the sleeve, but the retention of the loop is ensured by cooperation with the sleeve and the armature which completely envelops the loop.
French Pat. No. 1,581,324 discloses a rotor blade constructed of plastic material reinforced with fibers in the form of rovings which are longitudinal in relation to the axis of the blade, the rovings being impregnated with synthetic resin and disposed in loops in the blade root which is fixed to the hub of the rotor by bolts passing through the loops. The rovings are disposed in two hanks side by side in the drag plane, and each one of them forms, at the root of the blades, a loop or a pair of superposed loops, the two loops or pairs of loops being enveloped in a case consisting of a lower half and an upper half, which each comprise a ferrule projecting internally as far as half the height of the root of the blades, and which is engaged in each loop or pair of loops. Thus, the rovings of each loop or pair of loops pass around a cylindrical sleeve formed by the superposition or the alignment, in their axial extension, of two identical and coaxial, cylindrical ferrules traversed by the bolts for fixing to the hub.
French Pat. No. 2,004,934 likewise proposes, in order to connect a blade root to an armature for connection to a hub, cylindrical sleeves embedded in a core constructed of plastic material, of the blade spar and surrounded by fabrics or nappes of glass fibers encased in polymerized synthetic resin.
French Pat. No. 2,252,916 proposes, for the fastening of a blade root to a hub, an armature comprising a cylindrical sleeve equipped with an inner ring traversed by an arbor, the sleeve being integral with a cross-shaped tapered extension, in the form of a plate perpendicular to the sleeve at its center, and exhibiting ribs perpendicular to the plate and integral with the latter, on the side of the current part of the blade. This tapered and cross-shaped plate separates the hank of rovings forming the loop and which surrounds the sleeve into four parts separated from the sleeve and from the wings of the cross-shaped plate by laminated inserts of glass fabrics impregnated with resin to promote adherence and resistance to shear, filling elements being moreover provided in the voids delimited between the sleeve, the plate and the various parts of the hank, on the side of the current part of the blade.
French Pat. No. 2,327,858 describes a connecting element for the distributed transmission of forces in a component, the element comprising two sleeves which are coaxial and in axial alignment, each surrounded by a loop formed by a particuLar winding with one-half of a bundle of rovings of fibers impregnated with synthetic resin.
In this patent, which relates to the components constructed of plastic material reinforced with fibers and subjected to high loadings, each sleeve projects in relation to the end surface of the component, and the projecting part of the sleeve has a throat in which the fibers are wound. Furthermore, the loop surrounding the projecting part of the corresponding sleeve thins down to a thin disk, from the corresponding sleeve to the edge of the composite element, in such a manner that the high loads acting on the component are transmitted, without local concentration of stress, by a large surface to a connecting arbor traversing the sleeves. The rovings wound round are attached to the sleeves, but do not belong to the components, in such a manner that, if the latter comprises a filamentary structure, the rovings of the loops are not in any circumstances mixed with the filaments of the structure of the component. In fact, the connecting elements may be constructed in the form of prefabricated elements, each comprising sleeves with their winding and equipped with an internal ferrule, the assembly being prepared with the aid of a special tooling comprising a winding blank in rotation.
French Pat. No. 2,350,541 proposes a connection of a blade root constructed of composite material to a hub strap by bores formed in the blade root and traversed by fixing bolts, each bore being delimited by two cylindrical metallic sleeves which are in axial alignment and fixed by an elastic connecting component in an individual hole formed in the blade root. The internal ends of the two sleeves are separated from one another by a ring which is elastic and in contact with the latter, while the outer ends of the sleeves are in the planes of the opposite faces of the blade root, the two sleeves being coaxial and preferably identical.
French Pat. No. 2,390,331 describes a principal joint of a carrying wing system constructed of composite material having a matrix constructed of plastic material reinforced with fibers, for aircraft, the joint comprising a metallic shell exhibiting a central bore traversed by a principal articulation sleeve, and bordered on one side by a plurality of recesses in the form of niches in a circular arc, which are substantially concentric and divergent at their ends, to accommodate traction loops consisting of rovings. On the other side of the bore, the latter is bordered by a conical and composite compression body, between the branches of the loops proceeding out of the niches, said loops accommodated in the niches being incorporated in the structure of the wing system.
Moreover, it has likewise already been proposed, by U.S. Pat. No. 4,251,309, to connect the root of a blade to the hub of a rotor by four end loops, formed by composite bands and disposed in two pairs, symmetrical in relation to the flapping plane, of superposed and coaxial loops, a cylindrical and composite sleeve being engaged in each pair of loops, and an end cap being driven onto each sleeve portion accommodated in a loop.
Apart from the all-composite construction in this U.S. patent, and which has as a disadvantage that of multiplying the number of fastenings and thus of making the blade root very voluminous, all the other known constructions including cylindrical metallic sleeves exhibit a major and common disadvantage, which is added to the disadvantages specific to each construction, such as the presence of an elastic ring against which are applied the ends internal to the blade root of the two sleeves in French Pat. No. 2,350,541, the need to provide peripheral throats in the part of each of the two sleeves which projects in relation to the component to be attached in French Pat. No. 2,327,858, the construction of a single piece of the sleeve with a tapered and cross-shaped plate or with a shell exhibiting recesses in the form of niches in a circular arc, in French Pat. Nos. 2,252,916 and 2,390,331, respectively, and finally the cooperation of the metallic cylindrical sleeve or sleeves with a metallic armature or with a case consisting respectively of two half-shells or of a lower half and of an upper half completely enveloping the loop of the blade root, in French Pat. Nos. 1,544,344 and 1,581,324, respectively.
However, the principal disadvantage which is exhibited by all the constructions comprising metallic cylindrical sleeves arises from the fact that the sleeves used are constructed of steel and have a coefficient of thermal expansion very much greater than that of the composite materials which surround them. In molding, at a temperature which may, for example, be of the order of 120.degree. C., the metallic sleeves expand, especially radially, and the portion of the bundle of composite rovings which surrounds them at least partially is polymerized at the greater diameter of the expanded sleeves. On cooling, the sleeves contract, especially radially, in such a manner that a radial play is formed between the sleeves and the composite material which surrounds them; under the effect of the high stresses on the blade root (centrifugal force and alternating torques), this involves jarring of each sleeve in its laminated bore, and rapid damaging of the fastening. Furthermore, in the constructions in which the rovings are affixed around the sleeves by a film of adhesive, the contraction on cooling of a sleeve creates a tearing-off on the periphery, of the corresponding film of adhesive, and this tearing-off, while not completely rupturing this film of adhesive, does however initiate zones of separation. In this case, under the effect of the strong stresses on the blade root, as well as of the ageing by penetration of humidity into the initial separation sites, the film of adhesive deteriorates rapidly until there is total disintegration of the sleeve and of the blade root. In use, a separated sleeve is the cause of various instances of damage, especially the ovalization of the laminated elements which surround it; this makes it necessary to undertake a repair with a sleeve mounted with more play, further limiting the performance of such a repair. In the constructions with rovings affixed around the sleeves, another cause of the poor adhesion of the sleeve in the corresponding laminated bore results from a very weak pressure applying the film of adhesive against the sleeve, this resulting from the fact that the polymerization takes place in a mold which is closed and non-deformable and with laminated materials impregnated with liquid resin in the course of the polymerization.
In consequence, the principal disadvantage of the known fastening devices including cylindrical metallic sleeves is that the service life of the fastening is reduced to a large extent in service as a result of the radial play resulting from the differential expansions between the sleeves end part and the composite material which surrounds them, in the course of the polymerization. Furthermore, in the constructions including rovings affixed around the sleeves, the separation of the latter involves a still greater reduction of the service life of the fastening. Trials with a separated sleeve on tail rotor blades constructed of composite material have demonstrated the very marked influence of this reduction of the service life of the fastening on blades on which it had been possible to demonstrate initially an infinite service life with correctly affixed sleeves.